Numerous approaches have been taken in the design and construction of percussive drilling apparatus and particularly in the design of bits which employ multiple drilling teeth or drills for downhole drilling operations. Representative of approaches taken in the past is that disclosed in U.S. Letters Pat. No. 2,815,932 to Wolfram wherein a pneumatic hammer drives a generally fan-shaped arrangement of plungers with a pilot cutter positioned centrally of the plungers. Spring return members are employed in association with the plungers but are not in and of themselves capable of fully retracting the plungers after each blow. In U.S. Letter Pat. No. 2,595,126 to Causey, vertically adjustable inner and outer concentric drilling units are employed where one unit works ahead of the other to facilitate drilling a well. U.S. Letters Pat. No. 1,932,891 to Harner employs teeth arranged in fan-shaped rings which are successively reciprocated by pneumatic drive which operate cylinder heads. The arrangement is such that the teeth in one ring are driven between the teeth of another adjacent ring. In U.S. Letters Pat. No. 1,419,980 to Palma, fish-tail type cutting teeth are activated by divergently extending cylinders in cutting across a vertically extending arc. Similarly, in U.S. Letters Pat. No. 1,970,113 to Slawson, pressurized air is employed to drive a series of axially directed teeth; and in U.S. Letters Pat. No. 2,400,853 to Stilly, spring-loaded cutting tools are operated by fluid pressure.
It is proposed in accordance with the present invention to employ combustion chambers concentrically arranged to successively drive a series of teeth at the lower ends of concentric rings of pistons to deliver the necessary force to a series of teeth. The teeth are disposed in circumferentially spaced relation to one another in a series of concentric rows and are successively driven from the innermost to outermost row by sequential firing of the combustion chambers for each row or ring of teeth. Individual teeth are constructed so as to afford optimum wear and efficiency in operation.